When two wireless devices communicate with each other in a radio network in a traditional manner, each wireless device communicates radio signals with a serving base station of the radio network by sending uplink radio signals to the base station as well as receiving downlink radio signals from the base station. This is the traditional way of communication in a radio network also when the two wireless devices are located somewhat close to one another and being served by the same base station. This operation mode is commonly referred to as a cellular mode of communication. Recently, techniques have been developed to enable wireless devices in close proximity of one another to communicate radio signals with each other directly, i.e. direct communication, as controlled by the radio network and using frequency spectrum licensed to the network, such that each wireless device receives and decodes the actual radio signals that are transmitted from the opposite, or “peer”, wireless device. Bluetooth is another example of direct communication between wireless devices, although without control or involvement by any network and using unlicensed frequency spectrum.
Communication of radio signals may thus take place directly between the two wireless devices without the radio signals being communicated over the radio network via one or more base stations. In that case, the serving base station allocates radio resources, e.g. defined by time and/or frequency, which the wireless devices are allowed to use in the direct communication. Such direct communication between two wireless devices is commonly referred to as “Device-to-Device, D2D, communication” or terminal-to-terminal communication.
In the field of cellular radio technology, the term “wireless device” is usually used and will be used in this disclosure to represent any wireless communication entity capable of radio communication with a cellular radio network including receiving and sending radio signals. Another common term in this field is “User Equipment, UE” which is often used for various wireless devices such as e.g. mobile telephones, tablets and laptop computers. A wireless device in this context could also be a machine-to-machine type of device operating automatically such as a sensor, counter or measuring entity.
Further, the term “base station”, sometimes also referred to as a network node, radio node, e-NodeB, eNB, NB, base transceiver station, access point, etc., is used here to represent any node of a cellular radio network that is arranged to communicate radio signals with wireless devices. The base station described here may, without limitation, be a so-called macro base station or a low power base station such as a micro, pico, femto, Wifi or relay node, to mention some customary examples. Throughout this disclosure, the terms “network node” and “User Equipment, UE” could alternatively be used instead of base station and wireless device, respectively.
D2D communication may thus be employed whenever two wireless devices, also referred to as “peer devices” or just “peers”, are close enough to one another to be able to receive and decode direct radio signals from the opposite peer. Thereby, it may be possible to reduce the transmit power consumed and emitted in the area and also to reduce interference, as compared to what is required to enable a serving base station to communicate radio signals with the wireless devices in the traditional manner. Another advantage with D2D communication is that less radio resources are required than the conventional cellular communication. For example, two timeslots are needed for conveying a piece of data from one device to the other device in cellular communication while only one timeslot is needed for conveying the same piece of data in D2D communication. FIG. 1 illustrates various radio links that may be used for conveying data from a first wireless device D1 to a second wireless device D2. 100 denotes a D2D radio link between the devices D1 and D2, while 102a and 102b denote respective cellular radio links between the devices D1, D2 and a serving base station BS.
In a conventional cellular communication between a base station and a wireless device, a radio signal transmitted by the wireless device may be successfully received and decoded by the base station provided that the current radio conditions allow for sufficient quality of the received signals. This means that the received signal should not be too weak and/or interfered too much by other radio transmissions in the neighborhood for satisfactory reception and decoding. For example, the wireless device may be situated close to the cell edge and relatively far from the base station, or in a spot with bad radio coverage, so that the radio signal fades considerably on its way to the base station. Furthermore, the wireless device may in that case need to transmit with increased power in order to provide a sufficiently strong signal at the receiving base station, which may cause harmful interference to other nearby communications. Another possibility is to add redundant bits which can be used to assist the decoding in the base station's receiver although they occupy precious radio resources such that overall data throughput is reduced.
In a similar manner, when a D2D communication is employed between two wireless devices, a radio signal transmitted by one wireless device may be successfully received and decoded by the other wireless device provided that the current radio conditions allow for sufficient quality of the received signals. The requirements for successful reception and decoding are thus similar to the case of cellular mode discussed above. However, the signal quality in such a D2D communication with low power may not be sufficient for successful decoding of communicated data, and it may be required that at least one of the two communicating wireless increases its transmit power for favorable signal reception which in turn increases the power consumption and risk for interference.
It is thus a problem to achieve adequate signal quality and successful decoding of data when using D2D communication between two wireless devices, without causing too much interference and/or reduction of data throughput and/or power consumption, particularly under less than optimal radio conditions.